Investment casting, and more particularly, "lost wax" casting, is an industrial process used in the production of cast parts which involves: injecting wax into a metal die to produce a wax pattern; removing the wax pattern from the die; coating the pattern with a ceramic shell; heating the shell to melt, and thereby remove, the wax; and subsequently filling the ceramic shell with molten metal.
The investment casting process begins with the production of a one-piece, heat-disposable pattern. Patterns are usually made by injecting wax, foam, or plastic into a metal die. Upon cooling, the die is opened and the wax pattern is removed. The process is repeated until the desired number of wax patterns is produced.
Each pattern includes one or more gates, preferably located at the heaviest (thickest) portion of the pattern. Gates are generally flat wax projections which attach the pattern to a sprue. A sprue is a wax connector used to fasten patterns together to form a cluster. Sprues vary in size and shape and can accommodate from one up to several hundred patterns, depending on pattern size and shape. A flat soldering blade is placed between the gate of a pattern and the face of a sprue to heat seal the mating surfaces together. A pouring cup is secured to the central sprue of the cluster.
The cluster is dipped ("invested") in a ceramic slurry. Excess slurry is drained off and the cluster is coated with a fine ceramic sand and dried. The coating process is repeated, using progressively coarser grades of ceramic material, until a self-supporting shell is formed encapsulating the cluster.
The coated cluster is placed in a furnace or steam autoclave where the wax, including patterns, gates, and runners, melt and flow out of the mold through the pouring cup. The result is a ceramic mold corresponding to the shape of the cluster, each separate mold in the cluster having 3/16" to 3/8" thick walls and precisely shaped cavities corresponding to the desired configuration of a finished part. The ceramic mold is then fired to burn out the last traces of wax and to preheat the mold in preparation for the casting operation. The hot mold is taken from the furnace and molten metal is immediately poured into it.
After the metal-filled mold has cooled, the ceramic mold material is removed from the casting cluster by any suitable method, e.g., mechanical vibration or chemical cleaning. Individual castings are then severed from the cluster, for example, by cut-off wheels, and any remaining protrusions left by gates or runners are removed, for example, by belt grinding. The casting is then ready for secondary operations, including heat treating, straightening, machining, and whatever inspection may be required.
An important advantage of investment casting vis-a-vis other manufacturing processes is the high volume production which may be achieved from a single metal die through the use of wax patterns. For very large parts, a cluster may carry a single pattern. For smaller parts, a cluster may comprise several hundred individual patterns.
Another advantage of investment casting is the degree of complexity which can be imparted to the finished parts. Finished parts exhibit fine, precise detail without the complex machining operations normally associated therewith. In order to achieve this detail, however, it is important that the wax completely fill all voids in the metal die. Shrinkage of the wax patterns during cooling must also be minimized. Presently known methods of wax injection are not completely satisfactory in these respects.
Three principal techniques are commonly employed to inject wax into the metal die: semi-solid billet injection, paste injection, or liquid injection. The most common method is liquid injection.
Semi-solid injection begins with a cylindrical billet of wax, typically approximately seven inches in diameter and weighing approximately twenty-two pounds. The billet is softened to decrease the viscosity of the wax and thereby facilitate more complete penetration of wax into a die. Billets are typically softened by immersion in warm air or water for approximately 48 hours to achieve a uniform temperature. The softened billet is then placed in a shot chamber associated with a billet extrusion press. A shot chamber piston forces the semi-solid wax through an injection valve and into the metal die.
To facilitate loading, each billet occupies slightly less volume than its shot chamber. Moreover, each time a new billet is loaded into a shot chamber, small pockets of air are trapped between the irregular surfaces of successive billets, between the billet and the walls of the shot chamber, and between the billet and the shot piston. These conditions result in the introduction of air into the die. Trapped air causes air bubble surface or subsurface defects on the resulting wax pattern. Bleed holes are sometimes placed at carefully selected locations in the die so that unwanted air may blow through the die, but this technique is not completely effective because wax often plugs up the bleed holes, leaving internal pockets or holes within the pattern.
Moreover, billet wax injection results in considerable inefficiencies because of the fixed biller size. For example, if a particular die requires more than eleven pounds of wax for an injection, only one injection can be used per twenty-two pound billet. After injecting a die with wax from a first billet, the unused portion of that billet, which is insufficient to fill the die again, must be removed from the shot chamber. After a new billet is inserted, the wax which remained in the shot chamber must be purged to eliminate pockets of air at the old billet/new billet interface. Although the purged and discarded wax may be recycled, the associated time and labor costs are considerable.
The second commonly used wax injection method involves heating wax to soften it into a paste before injecting it into a die. Paste injection eliminates air pockets, and the need to purge unused wax, because: 1) there are no interfaces between successive fillings; and 2) the paste completely fills the shot chamber. A major disadvantage associated with this technique, however, is the tremendous handling problem posed by paste wax. Typically, paste injection requires an operator to manually scoop the paste into a shot chamber as needed. This results in increased time and labor per piece part. Transporting paste, maintaining its temperature, and depositing it into press shot chambers are cumbersome and inefficient procedures.
The third commonly used method involves heating the wax to a liquid state and thereafter injecting it into a metal die. The elevated temperature of liquid wax exacerbates shrinkage. Wax shrinks as a function of change in temperature. The higher the injection temperature of the wax, the more it shrinks as it cools to room temperature. Thus, for a given volume, paste wax shrinks more than semi-solid wax, and liquid wax shrinks more than paste wax as the wax cools to room temperature. As the wax shrinks, it pulls away from the mold surface. To achieve optimum dimensional integrity of the finished parts, such shrinkage must be minimized. Furthermore, maintenance of consistent, repeatable piece-part tolerances requires minimizing variations in change in temperature, since such variations will cause corresponding variations in shrinkage. Consequently, it is desirable to inject wax into the die at the lowest possible temperature while allowing the wax to completely fill all voids in the die while minimizing variations in the injection temperature in the course of successive injections.
The degree of precision with which the injection temperature of the wax is maintained is important in achieving optimal dimensional integrity of the finished parts. This is particularly true with respect to large parts because the amount of shrinkage is generally proportional to the size of the part.
Various systems have been proposed for controlling the temperature of a fluid (liquid or semisolid) substance. For example, maintaining the temperature of a substance within a vessel by enveloping the vessel in a water jacket is generally known. See, e.g., Mercer U.S. Pat. No. 3,788,522 issued Jan. 29, 1974; and Byrd U.S. Pat. No. 4,661,321 issued Apr. 28, 1987. However, reducing the temperature of heated wax with such a conventional heat exchanger poses insurmountable problems because the viscosity of wax increases rapidly as its temperature is reduced. Consequently, higher temperature (lower viscosity) wax tends to "tunnel" through lower temperature (higher viscosity) wax when the former is used to urge the latter through a heat exchanger. Tunneling occurs when wax at a higher temperature is forced against wax at a lower temperature in order to drive the lower temperature wax through a heat exchanger. Typically, the higher temperature wax, being less viscous than the lower temperature wax, creates small irregular paths through the lower temperature wax. The lower temperature wax remains essentially stationary as the higher temperature wax tunnels through it. Furthermore, the low coefficient of thermal conductivity of wax requires long residence times in conventional heat exchangers, which translates to increased piece part production costs.
Heat exchangers and pistons are generally known and have been used to propel liquids. See, for example, Scott U.S. Pat. No. 905,108 issued Nov. 24, 1908; Walsh U.S. Pat. No. 2,742,197 issued Apr. 17, 1956; Waring U.S. Pat. No. 217,965 issued Jul. 29, 1879, and Byrd U.S. Pat. No. 4,661,321 issued Apr. 28, 1987. However, such mechanical components have not been advantageously utilized in wax distribution systems.
An improved system for distributing wax to a series of injection presses is described in Christian U.S. Pat. No. 2,439,506 issued May 28, 1945. This patent discloses a system for distributing wax or other molding material, adapted to simultaneously supply a series of molding machines with the material in a proper condition for molding. Wax is pumped from a container and propelled through a conduit to a series of injection presses. In the conduit network, the temperature of the wax is maintained at a point where it is sufficiently plastic to flow. When the wax is injected into the mold, its temperature is preferably somewhat lower than its temperature while passing through the pump. This maintains the wax sufficiently viscous when entering the mold to avoid danger of mixing with the air in the mold cavity.
This system exemplifies some of the inherent problems associated with using wax as a pattern material. For example, it is desirable that wax completely fill the mold cavity in a condition free of holes. Furthermore, wax may be pumped only when it is in a liquid state. However, injecting wax into a mold in a liquid state necessarily requires using wax at a wax injection temperature which results in a high degree of shrinkage upon cooling.
Other attempts to deal with the shrinkage problem have been made. In the previously discussed paste injection method, wax has been cooled to a paste and scooped into a shot chamber associated with an injection press. This method, however, involves extensive labor costs and increases production time.
To the extent residence times can be minimized through the use of an innovative heat exchanger, the tunneling problem, which is inherent when a head of liquid wax is forced against a head of semi-solid wax, prevents high volume, continuous distribution of semi-solid wax. The present invention provides a wax distribution system which deals successfully with these problems.